Field of the Invention
The present invention relates to a system and method of detecting moving vehicles by means of ultrasonic energy and, more particularly, to a system and method for reliably detecting vehicles which travel through a vehicular traffic lane for counting the vehicles, classifying vehicles, opening access gates or the like.
Detecting, distinguishing and counting moving vehicles becomes particularly important in connection with the collection of fees at toll bridges, tunnels and toll booths along highways, parking facilities, etc. In the past, common devices which have been used to detect and count vehicles have included induction loop sensors and treadles. While induction loop sensors are relatively inexpensive and easy to install and maintain, they do have disadvantages when used for vehicle counting. Amongst the errors that are frequently encountered in the use of loop sensors is when two or more vehicles pass through a lane bumper-to-bumper. Loop sensors can only detect a single vehicle since they do not exhibit the sensitivity to distinguish vehicles in close proximity to each other. Additionally, certain types of vans with front engine drive or rear engine drive may be detected as one or two vehicles, depending on the body configuration and the distribution of the vehicle's mass. Many such loop sensors must also be "tuned" to roadway conditions, and errors may result due to humidity and temperature conditions which ambient conditions tend to de-tune loop sensors. Additionally, a single loop sensor cannot determine vehicle direction and, of course, loop sensors can be fooled by a mass of metal which can simulate a vehicle.
Treadles have, for the last 50 years, been the primary means used by U.S. toll authorities for counting vehicles. Treadles are, however, more expensive than loop sensors to install and maintain, and rubber pads used in connection therewith usually require replacement periodically which increases the cost of such treadles. When multiple strips are used in a treadle, these can provide extremely accurate counts and can be used to distinguish the directions of the moving vehicles. As suggested, treadles are expensive to install and maintain and must, under heavy traffic and bad weather conditions, be replaced approximately every one or two years.
Vehicle sensors utilizing ultrasonics have been proposed at least as early as 1934 in U.S. Pat. No. 1,982,341. In U.S. Pat. Nos. 3,046,522; 3,063,179; and 3,098,213, sonic vehicle sensors are disclosed which include transmitter/receiver pairs spaced from each other along a lane of traffic for effecting gate control. Such spaced transmitter/receiver pairs have also been used for establishing directionality of vehicles moving through a monitoring zone. See, for example, U.S. Pat. Nos. 3,109,157; 3,109,926; and 3,141,612. A cashiering control system for collecting fees for vehicles at a toll plaza disclosed in U.S. Pat. No. 3,317,892 utilizes overhead mounted ultra sonic sensor units in a similar way. For the most part, the foregoing patents utilize the ultrasonic sensors to sense the presence of a vehicle, although they are generally not adapted to distinguish between different types of vehicles or vehicles and non-vehicular traffic passing through the lane.
U.S. Pat. No. 4,187,487 discloses a moving object detection system which utilizes a single ultrasonic wave transmitter and receiver above the vehicular lane. A number of readings are taken to establish a standard plane which represents the actual delay of ultrasonic energy from a road surface. On the basis of a series of such computations, an average time is established. An arbitrary range is established so that if it takes the ultrasonic waves a predetermined variation from the average, the patentee assumes that the returning pulses within that time frame is a pulse returning from the ground plane or road surface. If the time for an ultrasonic pulse to return to the sensor is less than the ground plane reflection less another predetermined time factor, the patentee assumes that such pulse returns from a vehicle. Therefore, pulses received within a narrow window are all pulses which represent the presence of a vehicle, and a signal is generated representing vehicle detection. However, this technique can produce false readings, since it does not utilize a double sensor system and the passage of any object above a certain height would provide a different signal which can appear as a vehicle. The approach disclosed in this patent ignores the profiles of vehicles and does not take into account other objects passing through the detection area which can result in similar but false results.
In U.S. Pat. No. 4,674,069, a system is described for collecting and processing data relating to moving bodies wherein a single transmitter/receiver pair above the lane of vehicular traffic transmits an ultrasonic wave signal whose frequency is varied continuously. The wave reflected from the moving body is received and frequency demodulated to obtain a signal representing a frequency variation which is processed for producing a signal representing a feature of the moving body. However, because the data is obtained continuously, large quantities of data necessarily result which can, admittedly, be used to establish the contour of vehicles and to distinguish vehicles from one another. However, such a system entails substantial amounts of data and, consequently, requires significant memories not only for storing the incoming data but also the contours or profiles of vehicles which have been stored for comparison purposes. For most applications, however, such capability is unnecessary and too costly.
In U.S. Pat. No. 4,789,941, a computerized vehicle classification system is disclosed for classifying vehicles moving in a restricted vehicular traffic lane. Such passageways do not customarily involve the passage of people, or accommodate vehicles moving faster than approximately five miles per hour. The patentee discloses a method of taking successive, close-interval height readings and comparing them to pre-stored generalized maximums for the purpose of distinguishing passenger cars from trucks and buses. The patent discloses means for classifying vehicles by their number of axles, utilizing a wheel-actuated treadle mounted across the traffic lane. The vehicle classification system that is described involves measurement of vehicle height and the counting of axles, common requirements of fee control systems used by toll booth operators. The patentee discloses the desirability of filtering out uncharacteristic or aberational readings including maximum readings that are not sustained by the next momentary reading, as would occur in measuring an object protruding from a vehicle, for example, an antenna. Minimum heights, as would occur in measuring wind blown debris in a traffic lane are also filtered out. The patentee teaches the cooperative use of treadles to initiate or actuate the ranging process required to perform each classification cycle. The treadle, which relies upon the weight of the vehicle at its axle to trigger the start of the height measurement cycle, provides a positive means for event separation and detection of each vehicle. However, the patentee's proposal to utilize, as an alternative, only height measurements to provide the classification is insufficient to distinguish people moving through a passageway from vehicles moving through the same path. The proposed system only utilizes a single acoustic transmitter/receiver pair which is not adequate to establish the dynamic signature of one or more persons in motion, i.e. the motion of arms, legs and torso, which can easily be misread as vehicles. Counting errors, resulting from the use of this arrangement, are not easily correctable by the process of filtering by height measurements alone. Additionally, the single source ultrasonic input produces an insufficient volume of readings or data to count moving vehicles accurately. Moreover, such accuracy falls off sharply as the speed of passage through the detection area increases.